1. Field of the Invention
The invention relates to a method of determining a nuclear magnetization distribution from at least one magnetic resonance signal from a sub-volume of an object which is situated in a steady magnetic field, the sub-volume being selectively excited by means of a sequence comprising RF electromagnetic pulses and magnetic field gradients which are superposed on the steady magnetic field, resonance signals from at least a part of a spectrum associated with the sub-volume being suppressed.
The invention also relates to a method of shimming at least a part of a steady magnetic field in which a sub-volume of an object is situated, the sub-volume being selectively excited by means of a sequence comprising RF electromagnetic pulses and magnetic field gradients which are superposed on the steady magnetic field.
The invention furthermore relates to a magnetic resonance device for determining a nuclear magnetization distribution from at least one resonance signal from a sub-volume of an object, which device comprises means for exposing the object to a steady magnetic field and to a sequence of RF electromagnetic pulses and magnetic field gradients, the means being suitable for suppressing resonance signals from at least a part of a spectrum associated with the sub-volume, and means for detecting the resonance signal to be generated by means of the sequence.
2. Description of the Prior Art
A method of this kind is described in Journal of Magnetic Resonance 67, pp. 148-155, 1986. According to such a method an object is arranged in a steady magnetic field which is at least substantially uniform. The object is first exposed to a combination of a non-selective 90.degree. excitation pulse, a non-selective 180.degree. refocussing pulse, and a selective 90.degree. reset pulse. During the reset pulse a first gradient is applied which is superposed on the steady magnetic field. It is thus achieved that a magnetization which was oriented in the same direction as the steady field is reset, after refocussing, along the z'-axis after having been rotated to the transverse direction in a coordinate system x'y'z' which rotates at the so-called Larmor frequency and whose z'-axis coincides with the steady magnetic field; in other words, after application of the pulse/gradient combination it is achieved that longitudinal magnetization exists only within a slice of the object and not outside this slice. Outside the slice only dephased transverse magnetization will exist after some time. Subsequently, such a pulse gradient combination is applied two more times, be it with a second and a third gradient, respectively. The field directions of the gradients coincide with the direction of the steady magnetic field, the gradient directions extending perpendicularly with respect to one another. After the three pulse/gradient combinations, the magnetization in a, for example cubic sub-volume of the object will have been selectively longitudinally reset after having been transversally set. Outside the sub-volume only dephased transverse magnetization exists. By using so-called phase cycling, any spurious signals from outside the sub-volume are further suppressed. In order to obtain a magnetic resonance signal from the selected sub-volume by means of which, for example a spectrum of the sub-volume can be determined, a 90.degree. excitation pulse is generated. The resonance signal generated thereby is sampled and the spectrum is determined by means of Fourier transformation. In order to suppress, for example a water peak in the spectrum of the sub-volume, the latter 90.degree. excitation pulse may be replaced by a so-called 1-3-3-1 composite pulse as described inter alia in Journal of Magnetic Resonance 55, pp. 283-300, 1983, notably on page 298-299 of this article. The method is notably suitable for use in so-called wholebody magnetic resonance devices. Even though such a method offers suitable results, it is a drawback that a large number of RF electromagnetic pulses (at least 10) must be generated in order to obtain a magnetic resonance signal from a sub-volume of the object.